The ORDER BY can be fixed with:

   ORDER BY 1,3

or

   ORDER BY "Name", "IN Type"

When this is fixed, the other two errors mentioned in the original
comment above are shown.

This is because the constraint value passed in is a string, such as,
aggregate name (aggname) 'avg' and is being compared to a numeric
object id (oid) 16982.

The constraint value needs to be compared to the aggregate name
(a.aggname).  

Here is a working query to retrieve all aggregates:

SELECT a.aggname AS "Name", a.oid AS "Object Id",
 t.typname as "IN Type",obj_description(a.oid) AS "Comments" 
FROM pg_aggregate a, pg_type t, pg_user b 
WHERE a.aggbasetype = t.oid AND b.usesysid=a.aggowner

UNION

SELECT a.aggname AS "Name", a.oid AS "Object Id", '---' AS "IN Type", 
 obj_description(a.oid) AS "Comments"
FROM pg_aggregate a, pg_user b
WHERE a.aggbasetype = 0
AND b.usesysid=a.aggowner
ORDER BY "Name", "IN Type";

Here is a query to retrieve an aggregate supplied by a constraint value:

SELECT a.aggname AS "Name", a.oid AS "OBject Id",
   t.typname AS "IN Type", b.usename AS "OWNER",
   obj_description(a.oid) AS "Comments",
   a.oid AS "SQL"
FROM pg_aggregate a, pg_type t, pg_user b
WHERE a.aggbasetype = t.oid AND b.usesysid=a.aggowner 
AND a.aggname='avg'

UNION 

SELECT a.aggname AS "Name", a.oid AS "Object Id", '---' AS "IN Type",
   b.usename AS "Owner", obj_description(a.oid) AS "Comments", 
   a.oid AS "SQL" 
FROM pg_aggregate a, pg_user b
WHERE a.aggbasetype = 0
AND b.usesysid=a.aggowner
AND a.aggname = 'avg'
ORDER BY 1,3;